Fundamentally, starting circuits for high-pressure discharge lamps can be operated together with two sorts of ballasts: conventional ballasts (KVG) and electronic ballasts (EVG).
In the case of KVGs, it is mostly an iron-cored inductor which is used for current limitation of a started high-pressure discharge lamp, while the current limitation is controlled electronically in the case of EVGs.
Known high-voltage starting devices frequently use a circuit comprising a starting transformer whose secondary side is connected to the lamp to be started and whose primary side is connected to a circuit triggering the starting pulse. Such a circuit is disclosed, for example, in DE-A 195 31 622, in which the switch is implemented as a SIDAC, which for its part can be controlled via a transistor. A further example is the application PCT/IB96/00051, published under WO 96/25022. Here, as well, the switch is a SIDAC.
It is typically required in many countries as the standard for acceptably starting a high-pressure discharge lamp that the starting pulse is 2 .mu.s wide in conjunction with a voltage of 3.6 kV. This starting pulse is to be repeated with each system halfwave, and this corresponds to a repetition frequency of 100 to 120 Hz. A large inductance is required in order to generate such a starting pulse, and this necessitates large dimensions of the starting transformer. In addition, this stipulation is only a minimum requirement, which is necessary for successfully starting the high-pressure lamp.
The requirements made of a starting device with these specifications cannot in practice be fulfilled directly, since both in the case of conventional ballasts and, in particular, in the case of electronic ballasts, suitable inductances are high and the outlay on circuitry is not inconsiderable. Firstly, in the case of electronic ballasts it is necessary for a starting transformer to be small because of integration into a circuit. Secondly, said transformer may have only a small inductance, in order to avoid noise during remagnetization of a core in low-frequency operation (LF operation), and to deform the shape of the curve of the output voltage as little as possible, because in the case of electronic ballasts the shape of the curve of the output voltage is mostly that of a square wave. Thirdly, the price for such a starting transformer is relatively high. When generating the starting pulses by means of the switching elements used to date (mostly SIDACs), a wide scatter of the starting voltage occurs owing to tolerances in these switching elements, in particular in SIDACs. This fact is taken into account by purposely overdimensioning the switching elements or by specifically selecting the components used. Only relatively low repetition frequencies can be achieved using SIDACs, and moreover their response voltage is problematic.
If it is desired to drop below 2 .mu.s for the starting pulse width, in accordance with the above requirements the time between the individual pulses is to be shortened to less than 300 .mu.s, and this corresponds to a repetition frequency of approximately 3 kHz.
As experiments have shown, the switching elements, such as spark gap and SIDACs, so far employed in conjunction chiefly with EVGs for starting high-pressure discharge lamps are suitable only to a limited extent for such short starting pulses. It has been found that turning-on delays of up to 10 s occur when such switching elements are used for lamps which are difficult to start.